Diel vertical behavior of Copepoda community (naupliar, copepodites and adults) at the boundary of a temperate estuary and coastal waters

Despite a growing interest in diel vertical migration as a research topic, there are few studies in southern European marine coastal systems. This study determined the main structuring hydrological and physical factors at different temporal scales in copepod assemblage distribution patterns. Seasonal, tidal, lunar and diel vertical migrations accomplished by horizontal movements were examined on the main copepod fraction of the Mondego estuary, Portugal. Seasonal samples were conducted hourly at the mouth of the estuary, during diel cycles (25 h), both over neap and spring tides, at the bottom and surface, using a 63 μm and 335 μm mesh size nets. Simultaneously, four sites inside the estuary were sampled during flood tide to evaluate and compare copepods species' distribution along the estuary. Species life cycles were also categorized. Spring-spring tide best expresses the stable part of copepod-environment dynamics. Acartia tonsa and Oithona nana were distributed mainly at the bottom during ebb tides. A clear resident estuarine performance was noticeable in O. nana proving the estuarine preferences of the species. Neritic species showed preferences by saline waters, whereas the resident species were found mainly at estuarine areas. Copepodites stages showed a similar distribution pattern as estuarine species, avoiding leaving the estuary. In contrast nauplii and Oithona plumifera showed higher densities at surface flood tides. Indeed, vertical migrations accomplished by horizontal movements were mainly influenced by depth and tidal cycles, whereas day and night were not ecologically significant.